documentclass{article}
usepackage{tikz}
usepackage{tikz,pgfplots}

usetikzlibrary{calc,patterns,angles,quotes}

begin{document}
    begin{tikzpicture}[>=latex]
            begin{axis}[
    ticks=none,
    axis x line=center,
    axis y line=center,
    xlabel={$x$},
    ylabel={$y$},
    xlabel style={below right},
    ylabel style={above left},
    xmin=-2,
    xmax=2,
    ymin=-0.5,
    ymax=3]
    addplot [domain=-2:2,ultra thick] {2*x+1} node [pos=0.95, below left] {$C_f$};
    coordinate (A) at (0,1);
    coordinate (B) at (-0.5,0);
    coordinate (C) at (5,0);
    pic [draw, ->, "angle name", angle radius=1, angle eccentricity=1.5] {angle = C--B--A};
            end{axis}

    end{tikzpicture}
end{document}

documentclass{article}
usepackage{tikz,pgfplots}
pgfplotsset{compat=1.16}
usetikzlibrary{calc,patterns,angles,quotes}

begin{document}
    begin{tikzpicture}[>=latex]
            begin{axis}[
    ticks=none,
    axis x line=center,
    axis y line=center,
    xlabel={$x$},
    ylabel={$y$},
    xlabel style={below right},
    ylabel style={above left},
    xmin=-2,
    xmax=2,
    ymin=-0.5,
    ymax=3]
    addplot [domain=-2:2,ultra thick] {2*x+1} node [pos=0.95, below left] {$C_f$};
    coordinate (A) at (0,1);
    coordinate (B) at (-0.5,0);
    coordinate (C) at (5,0);
    pic [draw, ->, "$varphi$", angle radius=1cm, angle eccentricity=1.5]
    {angle = C--B--A};
            end{axis}

    end{tikzpicture}
end{document}